Central Noradrenergic System Inhibits Hyperpolarization-activated Currents in Proprioceptive Trigeminal Neurons

Objectives: Coordinated control of the masticatory system can be possible from its constant proprioceptive feedback. Proprioceptive trigeminal neurons are located not in the trigeminal ganglia but in the central nervous system to form mesencephalic trigeminal nucleus(MTN). The caudal population of MTN is found to intermingle with the LC nucleus. Locus coeruleus(LC), the major source of noradrenalin in the brain, makes it possible to adapt in stressful situations. G_iPCR activation, such as alpha 2A adrenoceptor(α2_A AR), is known to inhibit hyperpolarization-activated current(I_h)_. Given the proximity of MTN and LC, we aimed to investigate whether direct activation of LC neurons would inhibit I_h in MTN neurons.
Methods: SD rats(p14-20) were used for slice recordings. Whole-cell patch-clamp recording was used to obtain I_h from MTN neurons and to elicit action potential firings in single LC neurons. Microstimulation was used to stimulate multiple LC neurons. Changes in intracellular calcium levels(i.e. Ca²⁺ transients) were measured by Fura-2 based Ca²⁺ imaging during LC stimulation.
Results: α2_A AR on the membranes of MTN neurons were confirmed by immunohistochemical analysis. Application of α2_A AR agonist inhibited I_h by 49%(n=5, p=0.0006). I_h was reduced 36% by microstimulation(n=5, p=0.023) and 20% by single LC neuron activation(n=11, p=0.01). During microstimulation or single LC neuron activation, synchronized Ca²⁺ transients could be observed in the somatodendritic area of adjacent LC neurons in a time-locked manner. The recordings were further validated by TH positivity in stimulated neurons, therefore confirming that LC neurons were stimulated. I_h inhibition was reversed in the presence of a2_A AR antagonist(n=4) following LC stimulation.
Conclusions: Our results suggest that noradrenalin released by synchronized activation of LC induces I_h inhibition in MTN neurons. Altered membrane properties can lead to altered proprioceptive transmission from the masticatory system. Exaggerated activity of LC by prolonged stress may disturb appropriate proprioceptive feedback, thus leading to masticatory muscle dysfunctions.